The spatial resolution of the visual system is usually assessed using a simple measure of static visual acuity. A typical visual acuity test consists of a number of high contrast, black-on-white targets of progressively smaller size. The smallest target that one can successfully read denotes one's visual acuity. For example, if the smallest letters that you can read upon a Snellen Eye Chart subtend 5 minutes of arc (minarc) in height, you are said to have 20/20 (or “normal”) acuity. That is, the smallest letter that your visual system can successfully resolve is 5 minarc in height.
Visual acuity is a common measure of visual status because (1) it is easy to measure and (2) small amounts of refractive error in the eye yield marked declines in acuity test performance. Fortunately, most sources of refractive error are correctable via glasses or contact lenses.
However, recent findings have demonstrated that visual spatial processing is organized as a series of parallel—but independent—channels in the nervous system; each “tuned” to targets of a different size. As a result of this parallel organization of the visual nervous system, visual acuity measurements no longer appear to adequately describe the spatial visual abilities of a given individual. Modern vision research has clearly demonstrated that the capacity to detect and identify spatial form varies widely as a function of target size, contrast, and spatial orientation (see Braddick, Campbell & Atkinson, 1978, Handbook of sensory physiology or Olzak & Thomas, 1981, Journal of Optical Society of America, 71(1):64-70; Graham, 1989, Visual pattern analyzers. Oxford University Press, USA; De Valois & De Valois, 1988, Spatial Vision. New York: Oxford.). As a consequence, a simple assessment of visual acuity often does not predict an individual's ability to detect objects of larger size (Faye, 2005, Contrast sensitivity tests in predicting visual function. International Congress Series Vision 2005—Proceedings of the International Congress held between 4 and 7 Apr. 2005 in London, UK, 1282, 521-524; Ginsburg, Evans, Sekuler & Harp, 1983, Investigative Opthalmology & Visual Science, 24, p. 798-802; Ginsburg, 1984, A new contrast sensitivity vision test chart. American journal of optometry and physiological optics, 61(6), 403; Ginsburg, 2006, Current Opinion in Opthalmology, 17(1):19-26; Watson, Barlow & Robson, 1983, Nature, 302(5907):419-22).
Contrast sensitivity testing complements and extends the assessment of visual function provided by simple acuity tests. At the cost of more complex and time-consuming procedures, contrast sensitivity measurements yield information about an individual's ability to see low-contrast targets over an extended range of target size (and orientation).
Contrast sensitivity tests use sine-wave gratings as targets instead of the letter optotypes typically used in tests of acuity. Sine-wave gratings possess useful mathematical properties and researchers have discovered that early stages of visual processing are optimally “tuned” to such targets (Campbell & Robson, 1968, Application of Fourier analysis to the visibility of gratings. J Physiol, 197(3), 551-66; De Valois & De Valois, 1988, Spatial Vision. New York: Oxford; Watson et al., 1983, Nature, 302(5907):419-22).
A contrast sensitivity assessment procedure consists of presenting the observer with a sine-wave grating target of a given spatial frequency (i.e., the number of sinusoidal luminance cycles per degree of visual angle). The contrast of the target grating is then varied while the observer's contrast detection threshold is determined. Typically, contrast thresholds of this sort are collected using vertically oriented sine-wave gratings varying in spatial frequency from 0.5 (very wide) to 32 (very narrow) cycles per degree of visual angle.
Because high levels of visual sensitivity for spatial form are associated with low contrast thresholds, a reciprocal measure (1/threshold) termed the contrast sensitivity score is computed. The contrast sensitivity scores obtained for each of the sine-wave gratings examined are then plotted as a function of target spatial frequency yielding the contrast sensitivity function (CSF). Some typical CSF's are depicted in FIG. 1. Note the characteristic inverted-U shape of the CSF and its logarithmic axes.